The correct answer is E: transpires.

A potometer is an instrument used to measure the rate of water uptake by a plant shoot, which is an indirect way of determining the rate of transpiration. Transpiration is the process through which water is absorbed by the roots and then evaporates from the leaves, mainly through the stomata. The potometer measures how much water a plant absorbs, which is related to how much water is lost through transpiration.

Here's why the other options are incorrect:
- A. respires: Respiration is a process where plants convert glucose into energy. A potometer does not measure respiration.
- B. sucks air bubbles: While air bubbles may be used in setting up a potometer, the purpose is to measure water absorption, not air bubbles.
- C. loses weight: The potometer does not directly measure the weight loss of a plant.
- D. absorbs water: While water absorption is a part of transpiration, the potometer specifically measures the water loss due to transpiration, not just absorption.

Thus, the potometer is used to determine the rate at which a shoot transpires.